Lassa virus takes its name from the village in northern Nigeria where it
was first described. Small outbreaks of Lassa fever in humans have
also occurred in
Liberia, Zorzor, and various other parts of Nigeria, and
serological
evidence of infection has been documented in several other African
nations.

Lassa virus' natural reservoir is Mastomys natalensis, a rodent
prevalent throughout Sub-Saharan Africa. Many Mastomys species live
primarily
in and around homes in this region. Most human infection is thought
to be through aerosol transmission or contact with Mastomys
excretions and
blood resulting from capture and killing for consumption.

Of all of the diseases caused by arenaviruses, Lassa fever
has the greatest health impact. Severe Lassa virus infections occur in
adults of both sexes as well as children. The annual
number of cases of Lassa fever is estimated at 20,000-40,000,
with several
thousand fatalities. Overall mortality among hospital
cases is approximately 20%, usually following cardiovascular collapse. In
areas where ribavirin (see link:
Vaccination) and careful medical management are unavailable to
ameliorate the health impact, untreated Lassa fever can
be fatal to
60% of its victims. Lassa virus also causes unusually high fetal
mortality, with studies showing that fetal loss can be 92% if the mother
is infected early in pregnancy, and 75% in the last trimester. The
pregnant
woman also has an increased risk of death from Lassa fever.

Infection with Lassa virus leads to the gradual onset on fever and malaise
after an incubation period of about 10 days. As the course of the disease
progresses, severe prostration and systemic illness, with changes
in vascular permeability and vasoregulation, can occur. Bleeding is seen
in less than a third of those infected, but usually signals an unfavorable
prognosis. These most severe instances are known as
Lassa hemorrhagic
fever.
Lassa fever is the most exported of the viral hemorrhagic fevers, with
cases treated in the United Kingdom, the United States, Japan, Israel, and
The Netherlands.